As a result of noise, with respect to natural images input from a camera, scanner, or the like, typically half of the pixels in a pure black region may appear above zero brightness level and half below such level. When these images are printed or displayed, those pixels below the zero level due to noise perturbation are clipped to black, whereas those above zero reproduce with a shade of gray lighter than black in average with the black pixels, thereby destroying the purity of the black. This black shade clipping has further been typically known in the art to lower shadow contrasts and worsen the relative shadow grain.
In an effort to solve this problem, prior art solutions involve saturating the blacks beyond noise. However, this technique resulted in the obliteration of deep shadow contrast and detail.
Furthermore, if such natural images are to be stored, the negative brightness values pose a dilemma for the storage strategy. If negative states are allowed, this uses states to reproduce a brightness that cannot be displayed, however, if negative brightness states are not allowed, as is most common in the art, then part of the image information is lost as just described with the display or printing process.
Accordingly, a solution to the problem was needed of obtaining pure black without obscuring shadow detail.